The measurement and monitoring of intracranial pressure (ICP) for instantaneous (absolute) pressures as well as changes in pressure among patients with head injury, stroke oedema, idiopathic intracranial hypertension, hydrocephalus, papilloedema, acute intracranial haemorrhage and other conditions, provides necessary, and often vital information upon which medical and surgical treatment can be based. More recently, ICP has been found to be related to Glaucoma.
ICP is equivalent to intracranial cerebrospinal fluid (CSF) pressure and the latter appears to be equivalent to optic nerve subarachnoid space pressure when the pressure is greater than 0 mmHg. This subarachnoid space, containing CSF, surrounds the optic nerve up to the back of the eye. The classical theory of venous pulsation requires the presence of a gradient down the vein between the intraocular and retrolaminar optic nerve compartments so that the venous pressure is equivalent to intraocular pressure at its exit point on the disc surface. Intraocular pressure oscillations induced by the cardiac cycle leading to an intraocular pressure peak during systole were thought to cause a compressive force to act upon the venous walls at the exit and hence for intermittent collapse to occur in time with cardiac systole. The existence of a significant 7-13 mmHg pressure difference between the intra-ocular venous pulsation pressure and intracranial pressure has been a well-documented requirement for venous pulsation in healthy, normal dog, primate and humans. This pressure difference is thought to be due to central retinal vein resistance (narrowing), lamina cribrosa and other factors, and varies between individuals, becoming a major source of error when using ophthalmodynamometric methods to estimate ICP.
Currently, invasive techniques are used to measure ICP despite the many shortcomings of such practices. Continuous ICP measurement devices to monitor these conditions require a surgeon to drill a hole through the skull to implant transducers within brain tissue or to locate fluid connected tubes into the central brain ventricles. Intermittent measures can be obtained by needle puncture of the lumbar dura by spinal tap, to measure the cerebrospinal fluid (CSF) pressure. (CSF pressure and ICP are known to be equivalent so the terms are used interchangeably.)
Such procedures carry the risk of brain haemorrhage (up to 6%), malfunction, brain herniation and/or infection (up to 27%) and, furthermore, are expensive. Invasive ICP measuring devices comprise external ventricular drains (EVD) coupled to transducers and tissue microtransducers (e.g. Camino, Codman, Raumedic) all inserted through skull burr holes. Relevant diseases (described above) involve disorders of elevated ICP, but other disorders such as glaucoma, normal tension hydrocephalus and ventriculo-peritoneal shunt overdrain require ICP monitoring and are partly caused by low ICP.
Other non-invasive approaches have been proposed to estimate ICP, including using the combination or retinal arterial flow velocities and venous pulsation pressure (Cerepress), tympanic membrane displacement in the ear, ultrasonic detection of cranial pulsations, transcranial Doppler (TCD) ultrasonography of the middle cerebral artery, optic nerve sheath diameter and CT or MRI assessment of CSF volume. However, none of these has been shown to be sufficiently accurate at high ICP and none gives any useful measurements at low ICP.
Furthermore, existing non-invasive technologies have poor accuracy. For example, the tympanic membrane displacement method is based on acoustic stapedial reflex that, in theory, can measure intracranial pressure indirectly by measuring displacement of the eardrum since ICP is transmitted from the CSF to the perilymphatic fluid of the scala tympana in the labyrinth. However, this method has drawbacks due to the indirect nature of the measurement, poor accuracy and the necessity of having a patent, unobstructed cochlear aqueduct.
TCD ultrasonography provides a real-time spectral waveform of blood flow velocity in intracranial vessels. However, with many head injury patients, flow velocities in unilateral intracranial vessels may either increase or decrease due to vasospasms, loss of normal cerebrovascular auto-regulation or other reasons. Furthermore, other physiologic variables, such as cardiac output, pulse rate, hematocrit, positive end expiratory pressure (if ventilated) and carbon dioxide tension can alter TCD parameters. Accordingly, TCD ultrasonography cannot predict absolute ICP from instantaneous readings and, as a result, only trends can be inferred and, in any event, is difficult owing to the anatomic variability of the cerebral vasculature.
An ophthalmodynamometric method for estimating ICP was first described in 1925 by Baurmann. More recent techniques combine ophthalmodynamometry with reflectance oximetry of the retina or ultrasound measurement of blood flow in the central retinal artery (see US 2004/0230124), or automate the method by adding a camera and image processing software for detecting venous pulsations from a sequence of images of the eye fundus (see US 2006/0206037). However, the accuracy of ophthalmodynamometry combined with reflectance oximetry or central retinal artery flow appears little different from ophthalmodynamometry alone.
Classically, an ophthalmodynamometer has been used to apply force (ODF) on the eye, and elevate intraocular pressure (IOP), while an observer views the central retinal vein and notes the force when retinal vein pulsation just begins. The induced IOP is then calculated from the baseline IOP and ODF and termed the venous pulsation pressure (VPP). VPP determination is very subjective due to varying abilities of observers to detect the threshold at which veins pulsate. This adds one element of error to the measurement. Any automated method using blood column analysis suffers from the variation in human retinal vein anatomy, with there being markedly varying shapes and sizes of the retinal veins. Some more recent techniques rely upon detecting changes within the central retinal vein wall, but this is a small venous segment with great variation between individuals so both human judgement of its pulsation or machine judgement of size variation using threshold change detection is prone to wide variation and hence inaccuracy.